There have been increasing needs for a compact, thin type camera with excellent portability in a digital still camera market recently expanding. Circuitry parts for a signal process such as an LSI are becoming ever functionally sophisticated and downsized by means of minute wiring patterns. Also, storage media having a large capacity and in compact sizes are beginning to be available at low prices. However, imaging systems having lenses and solid-state imaging sensors such as a CCD or a CMOS are not yet sufficiently downsized, and there has been a demand for a development of a compact imaging system for realizing a camera with further portability.
In order to realize a downsized imaging system, a configuration exemplified by a lens array optical system having a plurality of micro lenses arranged on a flat surface is known. Conventional optical systems having a plurality of lenses arranged along an optical axis have problems that their volumes become large due to their lengths extending along the optical axis, and that aberrations thereof are large due to large lens diameters. On the other hand, a lens array optical system can be thin with respect to a direction of an optical axis, and aberrations thereof can be relatively reduced due to a small diameter of each micro lens.
Patent document 1 discloses an imaging system using such a lens array. The imaging system comprises, in order from an object side: a micro lens array having a plurality of micro lenses arranged on a flat surface; a pinhole mask formed of a plurality of pinholes, on a flat surface, having one on one correspondence with the micro lenses; and an image flat surface for a light having passed through each of the pinholes to form an image. Each micro lens forms a size-reduced image of an object on the pinhole mask, and each pinhole passes (sampling) a light of the size-reduced image which is different from pinhole to pinhole. Accordingly, an optical image of the object is formed on the image flat surface.
However, according to the imaging system of patent document 1, resolution of an optical image of an object formed on an image flat surface is determined by the number and density of micro lenses (namely, pinholes), and therefore, obtaining a high quality image has been difficult. In other words, because an arrangement of constituents each consisted of a micro lens and a pinhole pair determines a position of a sampling point for an image to be obtained, in order to obtain a high quality image, the number of the constituents needs to be increased so as to increase the number of sampling points, and a micro lens needs to be downsized so as to reduce pitch in arranging the constituents. However, due to limitation in downsizing of a micro lens, achieving a high resolution has been difficult. Also, a luminous flux reaching an image flat surface is limited by pinholes thereby losing a large quantity of light, and thus a problem in terms of sensitivity has been caused.
For solving the above problem, an imaging system using another lens array is disclosed in patent document 2. The imaging system comprises, in order from an object side: a micro lens array having a plurality of micro lenses arranged on a flat surface; a baffle layer including grid shape baffles for separating optical signals from the micro lens array such that the optical signals do not interfere each other; and a large number of photo-electric conversion elements arranged on a flat surface. An imaging unit comprises a micro lens, a space corresponding to the micro lens and separated by the baffle layer, and a plurality of photo-electric conversion elements corresponding to the micro lens.
In each imaging unit, a micro lens forms an optical image of an object on a corresponding plurality of photo-electric conversion elements. Accordingly, a shot image is obtained in each imaging unit. Resolution of the shot image corresponds to the number of photo-electric conversion elements (the number of pixels) configuring one imaging unit. Because relative positions of micro lenses with respect to the object differ from each other, a position of the object's optical image formed on the plurality of photo-electric conversion elements is different from imaging unit to imaging unit. As a result, an obtained shot image differs from imaging unit to imaging unit. Through signal processing a plurality of shot images different from each other, an image can be obtained. With this imaging system, a quality of a shot image obtained from an imaging unit is low due to the small number of pixels included in each imaging unit. However, by signal processing using shot images slightly displaced with respect to each other each obtained from each of a plurality of imaging units so as to reconstruct an image, it is possible to obtain an image with a quality similar to that obtained when shooting with a large number of photo-electric conversion elements.
[patent document 1] Japanese Examined Patent Publication No.59-50042[patent document 2] Japanese Laid-Open Patent Publication No.2001-61109